In recent years, the motors that drive the fans of outdoor units of air conditioners (hereinbelow, abbreviated as outdoor fans) have been controlled by inverters, wherein rotational speed is controlled in accordance with instructions regardless of the external load's size. Because outdoor fans are generally disposed outdoors, natural wind sometimes causes them to rotate in reverse when stopped; when the motor is started up in such a state, an overcurrent is sometimes generated in the inverter circuit, which causes the motor to stop abnormally. One way of solving this problem is a method, which has already been disclosed (e.g., refer to Japanese Patent Application Publication No. H8-303386), that presets a permissible start-up rotational speed at which the fan can be started up even if it is rotating in reverse; with this method, if the fan is rotating in reverse at a speed greater than or equal to that start-up permissible rotational speed, the fan is not started up.